Added a new parameter to sip_trace() function which is
the tracing type. This can be m(message)/t(transaction)/d(dialog).

Transaction level tracing is done by registering REQUEST_SENT,
REPLY_RECIEVED and REPLY_SENT callbacks when sip_trace() is called.
Transaction module has to be loaded and the function call has to
be made from a request route.

Dialog level tracing is a little more complicated. INVITE transaction
is traced the same level as a transaction traced with 't' flag. For
the other transactions DLGCB_CREATED callback is used to know when
dialog is created and callbacks for the rest of the dialog can be
created. Correlation_id has to be carried all the way to this
callback which is called after all the routes are called. In order
to pass the correlation_id from sip_trace() to DLGCB_CREATED
correlation_id has been saved into an avp. Also, since DLGCB_CREATED
is called for all dialogs and some users may opt out not to trace
some dialogs, FL_SIPTRACE message flag has been added to signal
from sip_trace() to the callback whether this dialog should be
traced or not.

Moreover FL_SIPTRACE flag is used to signal whether stateless
replies shall be traced, such as the 200 OK generated after a
successful registration.

The implementation is not complete, some messages such as the
180 and negative acks will be missing. Moreover internally
generated BYE transacitons at timeout might be missing but
these shall be fixed in future commits.